1. Field of the Invention
The present invention relates to an iris diaphragm device for use in a projection high definition television (HDTV) system or in a video projector.
2. Description of the Related Art
In a projection high definition television (HDTV) system or a video projector, generally, a lamp with a high intensity is used as a light source and the amount of light is reduced as required, so that the brightness of a display image can be adjusted according to use environment. If an electric power applied to the lamp is reduced to 70% of the rated power or below in order to reduce the light amount of the lamp, the life of the lamp is shortened. So, an iris diaphragm device to mechanically block the light from the lamp is widely used so that the light amount of the lamp can be reduced to a desired level without shortening the life of the lamp. Included in such iris diaphragm devices are: a slide mechanism type in which two diaphragm blades are provided to slide parallel to each other and the blocked area of the lamp optical path is adjusted according to the change of the overlapping area of the two diaphragm blades; and a rotary mechanism type in which two diaphragm blades supported respectively on two parallel rotary shafts to rotate synchronously with each other are caused to revolve with respect to the respective rotary shafts and the blocked area of the lamp optical path defined between the two diaphragm blades is adjusted by the two diaphragm blades revolving with respect to the respective rotary shafts (refer to, for example, Patent Document WO2005-026835).
In the iris diaphragm devices as described above, it is essential to control the opening between the two diaphragm blades at a high speed and also with a high accuracy to thereby assure a rapid response to image scene change. In the rotary mechanism type iris diaphragm device, since the opening between the two diaphragm blades is adjusted according to the rotation angle of the rotary shafts of the two diaphragm blades, a magnetic sensor is provided which precisely measures the rotation angle of the two rotary shafts and precisely controls the rotation angle of a motor.
FIGS. 3 and 4 show the structure of the relevant portions of diaphragm blades 112 and 114 of a conventional rotary mechanism type iris diaphragm device 10. In an example of FIG. 3, a magnetic sensor 16 includes a Hall effect device 18 as a plate-like magnetic sensing portion. A magnet 20 as a sensed portion is fixed to a side wall of a yoke 22 having a square C shape cross section, and a facing surface 18a of the Hall effect device 18 and a facing surface 20a of the magnet 20 are arranged parallel to an axis line 24C or 26C of the rotary shafts of the diaphragm blades 112 and 114 as shown in FIG. 4A and 4B thus forming a “vertical composition”.
The yoke 22 is fixed to either a rotating portion of a motor 28 or a portion (specifically, a gearwheel fixed to a motor rotary shaft in the example shown in the figure) 30 rotary-driven directly by the motor rotary shaft, the diaphragm blade 112 also is fixedly attached to the aforementioned gearwheel 30, and therefore the rotation angle change of the rotary shaft of the diaphragm blade 112 coincides with the angle change of the facing surface 20a of the magnet 20. On the other hand, since the Hall effect device 18 is mounted on a circuit board 32, and the circuit board 32 is fixed to a supporting member such as a bracket, the angle of the facing surface 18a of the Hall effect device 18 does not change if the diaphragm blades 112 and 114 revolve. The revolution angle of the diaphragm blades 112 and 114 can be figured out based on the fundamental principle of the Hall effect device 18 that only a magnetic field component perpendicular to the device surface (the facing surface 18a) is sensed and that an output voltage of a value corresponding to the strength of the magnetic field sensed is generated.
In this connection, since the gearwheel 30 meshes with a gearwheel 36 fixed to a rotary shaft 26 of the diaphragm blade 114, the power of the motor 30 is transmitted to the rotary shaft 26 via the gearwheels 30 and 36, and the diaphragm blades 112 and 114 are caused to revolve synchronously with each other. The magnetic sensor 16, the circuit board 32 and the gearwheels 30 and 36 are located so as not to interfere with a lamp optical path L.
In the rotary mechanism type iris diaphragm device 10 described above, the two diaphragm blades 112 and 114 are supported respectively on the two parallel rotary shafts adapted to rotate synchronously and revolve with respect to the axis lines C24 and C26 of the rotary shafts, whereby the blocked area of the lamp optical path L located between the two diaphragm blades 112 and 114 and oriented orthogonal to the axis lines C24 and C26 can be adjusted, but since the area influenced by the heat and light from the lamp light is larger than the area of the lamp optical path L defined, it can happen that the Hall effect device 18 and the circuit board 32 are deteriorated by the heat and the light. Also, the magnet 20, in order to reduce the influence of thermal demagnetization, is inevitably forced to be expensive. And, the heat deterioration of the Hall effect device 18 and the magnet 20 makes it difficult to prevent deterioration of the angle detection accuracy.